FIG. 14 is a schematic drawing showing a structure of a liquid crystal display device including a liquid crystal panel 10 of the active-matrix variety with thin-film transistors (“TFT” hereinafter) as the switching element, and drivers (source driver 12, gate driver 14) for driving the liquid crystal panel 10. The liquid crystal panel 10 includes a plurality of source bus lines 16 which are disposed parallel to one another in a vertical direction of the screen, and a plurality of scanning lines 18 which are disposed parallel to one another in a horizontal direction of the screen. Outside of the liquid crystal panel 10, the source bus lines 16 are connected to the source driver 12, and the scanning lines 18 are connected to the gate driver 14. The source bus lines 16 and the gate bus lines 18 are substantially orthogonal to one another, and areas corresponding to intersections of the source bus lines 16 and the gate bus lines 18 make up pixels. Each pixel includes a TFT 20 and a liquid crystal cell 22.
In order to display an image on the liquid crystal panel 10, the source driver 12 applies tone voltages according to tone data (image data) of their respective scanning lines 18 to the corresponding pixels of the scanning lines 18, while the gate driver 14 successively switches ON TFTs 20 of the respective the scanning lines 18.
FIG. 15 is a circuit diagram showing an equivalent circuit of a pixel in the liquid crystal panel 10 of FIG. 14. The liquid crystal cell 22 is connected to the drain of the TFT 20 and to a common electrode 26 which is common to all pixels. Further, though not shown in FIG. 14, the pixel has a load capacitor 24. The load capacitor 24 is connected to the drain of the TFT 20 and to a load capacitor electrode 28 which is common to all pixels.
To activate the pixels, a tone voltage according to tone data is applied to the liquid crystal cell 22 from the source bus line 16 while the TFT 20 is ON. The tone voltage, which is set according to tone data, is applied to each pixel according to the tone data for every frame. In response to an applied voltage, the liquid crystal molecules in the liquid crystal cell 22 undergo changes by their dielectric anisotropic property to change directions of a long axis (director). Since the liquid crystal molecules are optically anisotropic, a change in direction of the director causes a change in polarization direction of light which travels through the liquid crystal cell 22. The quantity of light through the liquid crystal cell 22 is controlled by tone voltages applied to the liquid crystal cell 22 with the aid of other members of the liquid crystal cell 22, such as polarizers. The luminance of pixels is so controlled to attain desired tone luminance to be displayed, thus displaying images.
The tone voltage applied to the liquid crystal cell 22 is also applied to the load capacitor 24. The load capacitor 24 stores charge according to the applied tone voltage. The charge stored in the load capacitor 24 is maintained therein even after the TFT 20 is switched OFF (gate OFF) until the tone voltage is applied again in the next frame. In this way, the tone voltage applied to the liquid crystal cell 22 remains over one frame period.
In changing tone luminance of pixels between frames, the direction of the director of the liquid crystal molecules displaying the tone luminance of the frame before the change is changed with application of a tone voltage to the pixels of the next frame. This brings about a change in optical characteristics of the pixels, thereby changing tone luminance of the pixels.
Incidentally, it takes some time for the liquid crystal molecules to respond to a change in tone voltage. For example, the response speed of nematic liquid crystal is on the order of several ms to several ten ms depending on display modes. This means that the response of the liquid crystal molecules does not complete at the OFF of the TFT 20, and the director keeps changing even after the TFT 20 is switched OFF.
Here, the liquid crystal molecules has a dielectric anisotropic property, and a change in director of the liquid crystal molecules inevitably changes the dielectric constant of liquid crystal in the liquid crystal cell 22, which in turn changes the capacitance (electric capacitance) across electrodes of the liquid crystal cell 22. As noted above, the director of the liquid crystal molecules keeps changing even after the TFT 20 is switched OFF, while supply of charge to the liquid crystal cell 22 and the load capacitor 24 is stopped at the OFF of the TFT 20. Thus, a change in capacitance of the liquid crystal cell 22 after OFF of the TFT 20 causes a voltage change across the electrodes of the liquid crystal cell 22. That is, the voltage of the liquid crystal cell 22 becomes different from the tone voltage which was applied while the TFT 20 was ON after the TFT 20 was switched OFF.
Thus, despite the characteristics of the liquid crystal molecules which can respond within one frame, there are cases where the tone luminance obtained may become different from that desired for display due to the voltage of the liquid crystal cell 22 which changes within one frame. Conversely, in order to obtain desired tone luminance, there are cases where the same voltage needs to be applied over several frames (e.g., 3 frames).
Note that, a technique for correcting the slow response of liquid crystal molecules in response to an applied voltage is disclosed, for example, in Japanese Unexamined Patent Publication No. 10299/1989 (Tokukaisho 64-10299) (published date: Jan. 13, 1989). However, the technique disclosed in this publication incorporates a correction circuit in the device to predict data every time it is outputted. This requires a complex structure for the correction circuit and poses the problem of slow processing speed. Further, the data inputted to the correction circuit include data which was corrected immediately before in the correction circuit and stored in a memory. In other words, the corrected data is used to further correct the next data. This requires a complex device structure. Further, a technique such as that disclosed in the foregoing publication does not take into consideration the foregoing drawbacks caused by the capacitance change of the liquid crystal cell 22, and the foregoing publication does not teach a specific method of converting the data to solve these drawbacks.